The present invention relates to an electric power tool.
There are many power tools, such as electric drills, that are cordless to improve convenience and operability. A cordless power tool uses a rechargeable battery (battery pack) such as a lithium ion battery as a power supply for driving a motor.
Japanese Laid-Open Utility Model Patent Publication No. 5-49283 describes an electric power tool provided with an illumination means to illuminate dark places so that the power tool can be used even in a dark place. The electric power tool includes a first switch for operating the electric drill (driving the motor) and a second switch for lighting the illumination means (penlight), which is arranged on the electric drill body. The second switch is operated independently from the first switch.
When switching on the first switch to drive the electric drill (motor), the second switch is switched off to deactivate the illumination means if light is sufficient, and the second switch is activated to activate the illumination means if light is insufficient.
In order to protect the rechargeable battery from over-discharging, this type of electric power tool includes a function for stopping the motor if the voltage value of the rechargeable battery reaches a predetermined reference voltage value when the motor is driven. The reference voltage value is acquired from the discharge characteristics (battery voltage relative to discharge capacity) of the rechargeable battery when the motor is driven.
Therefore, in the electric power tool of Japanese Laid-Open Utility Model Patent Publication No. 5-49283, the rechargeable battery is protected from over-discharging by using the reference voltage value acquired from the discharge characteristics of the rechargeable battery under the condition that the motor is driven.
When only the illumination means is activated, the illumination means is deactivated if the voltage value of the rechargeable battery reaches the reference voltage value, which is acquired from the discharge characteristics of the rechargeable battery under the condition that the motor is driven, to protect the rechargeable battery from over-discharging.